1. Field of the Invention
The present invention relates to a feed-forward amplifier, and in particular, to a feed-forward amplifier including an error detection loop that has a predistortion compensator in an input path to a main amplifier to detect a distortion component in the main amplifier; and an error rejection loop that injects into the output of the main amplifier the distortion component detected by the error detection loop so as to remove the distortion.
2. Description of the Prior Art
An example of such a feed-forward amplifier is disclosed in Japanese Patent Application Laid-Open No. Hei 1-200807, and FIG. 4 shows a block diagram of this amplifier. Referencing FIG. 4, this amplifier has an error detection loop 100 that detects a distortion component in a main amplifier 106 and an error rejection loop 200 that injects the detected distortion component into an output signal path 114 from the main amplifier 106 to remove the distortion.
The error detection loop 100 has a predistortion circuit (a predistortion compensator) 120A at the input of the main amplifier 106 to minimize the distortion in the main amplifier 106 and also has on a delay line 109 an attenuator 108 for controlling the error detection loop 100.
A signal input from an input terminal 101 is divided by a power divider 103, and one of the outputs from the power divider is input to a power combiner 104 through the predistortion circuit 120A and main amplifier 106. In addition, the other output from the power divider 103 is input through a variable attenuator 108 and a variable delay line 109 to the power combiner 104, in which it is combined with an output from the main amplifier 106.
The same component as in the main amplifier 106 is supplied from the power combiner 104 to the output signal path 114 from the main amplifier, and the distortion component in the main amplifier 106 detected by the error detection loop 100 is output to an error injection path 115. A signal on the error injection path 115 passes through a variable attenuator 110 and a variable delay line 111, and is amplified by an auxiliary amplifier 107 and then combined with the signal in the output signal path 114 from the main amplifier by the power combiner 105. The output from the power combiner 105 is output from an output terminal 102.
In addition, FIG. 5 shows another conventional example wherein like components have the same reference numerals as in FIG. 4. In this example, the error injection path 115 is input to the auxiliary amplifier 107 through the variable attenuator 110, the variable delay line 111, and a predistortion circuit 120B. The predistortion circuit 120B reduces the distortion in the auxiliary amplifier 107. The remainder of this configuration is the same as in FIG. 4.
These conventional examples have the following problems. First, if the gain of the main amplifier 106 varies due to a variation in the ambient temperature, a corresponding variation in output level causes the amount of distortion occurring in the main amplifier 106 to vary but this distortion cannot be compensated for due to the constant distortion compensation amount of the predistortion circuit 120A.
In general, in the feed-forward amplifier, if the gain of the main amplifier 106 varies due to a variation in the ambient temperature, the variable attenuator 108 interposed into a linear signal path 113 must be adjusted so that the signal path 112 from the main amplifier and the linear signal path 113 have the same gain and opposite phases, in order to extract distortion correctly. The feed-forward amplifiers in FIGS. 4 and 5, however, have the variable attenuator 108 in the linear signal path 113, so the output level of the main amplifier 106 remains changed.
Second, if the gain of the auxiliary amplifier 107 varies due to a variation in the ambient temperature, the compensation amount of the predistortion circuit 120B varies to prevent the distortion in the auxiliary amplifier 107 from being compensated for.
In general, in the feed-forward amplifier, if the gain of the auxiliary amplifier 107 varies due to a variation in the ambient temperature, the variable attenuator 110 is controlled so that the output signal path 114 from the main amplifier and the error injection path 115 have the same gain and opposite phases, in order to extract distortion correctly. In the conventional feed-forward amplifier with the variable attenuator 110 preceding the predistortion circuit 120B, however, the above operation causes the input level of the predistortion circuit 120B and thus the compensation amount to vary.